In the area of energy conservation and the processes where heat transfer mechanism plays an important role, emittance of the material is an important parameter, which should be known accurately and precisely for its optimization. The value of the emittance of a material at any temperature is, generally, derived from the ratio of the irradiance of the material and that of the blackbody maintained at that temperature of the material. In the mid-infrared region, for the determination of the emittance of a material at room ambient this method is not successful because of low signal to noise ratio.
The determination of diffuse reflectance is important because of its day to day applications in terms of determining    1. Quality of fruits/vegetables.    2. Fat content in milk.
The measurement of diffuse reflectance can also be used in studying:    1. Influence of computer key boards on visual functions and    2. Quantification of brain endema.
Moreover, diffuse reflectance study is the effective tool for the following fields:    1. Remote sensing: by monitoring diffusely reflected radiation from remote objects/targets.    2. Forensic Science: by maintaining the data bank on automobile paint, etc.,    3. Energy conservation material analysis: Developmental work on designing new materials.    4. Pollution monitoring: by monitoring the industrial & automobile exhaust for the pollution.    5. Fire Proof material analysis: developmental work on uniforms for the fire fighting units.    6. Military Application: Applications like creating the signature of the target for passive IR systems.
All these things show the importance of the work to be carried out.
The Applicants have found that if the amount of energy of specular reflectance (Rs) and diffuse reflectance (Rd) and also the amount of energy of regular transmittance (Ts) and diffuse transmittance (Td) are determined, the total amount of energy absorbed may be easily derived from the following equation:Absorptance=1−[Rs+Rd+Ts+Td]
Here, (Rs+Rd) is the reflectance value and similarly (Ts+Td) is the transmittance value. According to Kirchhoff's law, the absorption of a material is equal to its emittance. Hence the above relation can be written as:Emittance=1−[Rs+Rd+Ts+Td]
Since this relation is more suitable for the spectral region of 2.5 μm to 15 μm, it seems to be a method for precise determination of absorptance or emittance of materials at ambient temperature. For opaque coatings, the contributions from diffuse and regular components of transmittance are zero and for opaque coatings on a rough surface, the contribution from regular reflectance is also zero. Thus for a case of opaque coating on a rough surface, the diffuse reflectance will only contribute to emittance.
Though, theoretically it seems simple but experimentally the determination of diffuse reflectance/transmittance would have been possible only after the invention and availability of efficient diffuse coating material, Fourier Transform Infrared Spectrophotometer and the sensitive, fast, low noise detectors.
References may be made to the article entitled “Diffuse reflectance spectroscopy: a comparison of the theories” by E. L. Simmons, Applied Optics vol. 14 (1975), wherein all the existing models are discussed in a detailed way to handle the diffuse reflectance problem. Even though details about the principle and intricacies are described, no computational procedure is reported.
Another reference may be made to an article titled “Optical properties of powders. Part I. Optical absorption coefficients and absolute value of diffuse reflectance. Part II. Properties of luminescent powders” by N. T. Melamed, Journal of Applied Physics vol. 34 (1963) gives an account of the theory developed by him to solve the diffuse reflectance problem. There also no specific computation is discussed.
Yet another reference may be made to an article titled “An Article on Optics of Paint Layers” by Kubelka-Munk. The article gives an idea about the differential equation method. It discusses the theoretical treatment of the optics of coatings.
Still another reference may be made to an article titled “Measurement of total reflectance, transmittance and emissivity over the thermal IR spectrum” by F. J. J. Clarke and J. A. Larkin. The article discusses about the diffuse reflectometer/transmissometer that can determine the diffuse reflectance/transmittance of any type of sample.